Thermoplastic elastomers are a special class of polymers of practical and theoretical interest. They were introduced by the Shell Chemical Company (U.S.A.) in 1965. Thermoplastic elastomers are novel because they can be formed into useful articles by common, rapid, thermoplastic processing techniques. They exhibit rubber-like properties such as high resilience, high tensile strength and reversible elongation. Thermoplastic elastomers consist of block copolymers of the A--B--A structure, --(A--B).sub.n -- structure or even --[B.sub.x --C(A)--].sub.n --, where A is a polymeric segment having a high glass transition temperature and B is an elastomeric polymer segment, n is the number of repeating block sequences in the polymer, x is the number of repeating units of one monomer in the polymer and C represents a graft point in the polymer backbone. There are four factors which influence the elastomeric performance of these polymers. They are choice of monomers, block lengths of A and B, weight fractions of A and B, and the molecular weight distribution of the elastomeric B block.
Typically, thermoplastic elastomers are formed by sequentially incorporating aromatic monomers, such as styrene, and dienes, such as butadiene or isoprene, into polymer chains using anionic polymerization chemistry. This results in A--B--A triblock copolymers which have polybutadiene and/or polyisoprene and/or other unsaturated blocks present in the center (B) portion. It is also known to substitute other polymers for the polystyrenic end blocks thereby synthesizing polymers with end-blocks consisting of poly(.alpha.-methylstyrene), poly(4-vinylbiphenyl), poly(styrene-co-1,1-diphenylethylene), poly(styrene-co-.alpha.-methylstyrene) and poly(vinyl naphthalenes).
The unsaturation present in the center blocks is undesirable for applications in which good chemical resistance is required. For example, the unsaturation causes the polymers to have limited thermal stability and poor resistance to atmospheric oxygen and ozone. The polymers are also susceptible to halogens and strongly acidic materials, due to the high reactivity of the unsaturated units in their center blocks toward such reagents. Limited chemical stability is a well-recognized deficiency of all polymers with unsaturated repeating units, especially those derived from diene monomers.
Polymers containing unsaturated monomer units are often hydrogenated to obtain materials that have improved thermal and chemical stability. For example, the polymer that results from butyl lithium initiated polymerization of butadiene contains 1,4-butadiene and 1,2-butadiene repeating units and it reacts easily with oxygen, ozone and other chemicals. When this polymer is hydrogenated, it becomes a copolymer of ethylene and butene-1. This polymer has much better chemical resistance than the parent polybutadiene. Similarly, hydrogenation of polyisoprene yields a copolymer containing ethylene and propylene repeating units. This polymer also has much better chemical resistance than the parent polymer. Hydrogenation of unsaturated units in copolymers is also an important way to improve chemical and thermal stability. Hydrogenation of a statistical styrene-butadiene copolymer, for example, yields a polymer containing styrene, ethylene and butene-1 units. Of special interest today is the product obtained by hydrogenating the butadiene repeating units in butadiene-acrylonitrile copolymers. The resulting polymers have excellent chemical and solvent resistance and are useful in seals and gaskets.
Hydrogenation of the unsaturated blocks in thermoplastic elastomers, such as the Kraton series of polymers has resulted in commercial products (e.g., Kraton.RTM. G-1652) that have improved chemical resistance compared to their unhydrogenated counterparts, making them and their analogs important commercial products for this very reason. The chemical stability of the hydrogenated products is sufficiently high to permit chemical modification of the blocks containing the aromatic monomers (e.g., polystyrene) to obtain thermoplastic elastomers with increased softening points and other useful characteristics. Such chemical modifications are not possible with the parent unsaturated polymers.
Hydrogenation of unsaturated units in polymers is catalyzed by metals or metal salts and is an equilibrium process. Although the equilibrium between saturated and unsaturated units heavily favors the formation of hydrogenated units, the reaction cannot be conducted completely. When the reaction is conducted under conditions that are commercially practical, the hydrogenated polymers can contain as much as one (1) percent residual unsaturation. This presence of such residual unsaturation adversely affects properties such as oxidative resistance and ozone resistance. It also creates difficulties when attempts are made to modify thermoplastic elastomers by nitration or arylsulfonylation.
For example, when attempts are made to nitrate or arylsulfonate the styrene units in Kraton.RTM. G-1652, the molecular weights of the polymers rapidly increase, reaching values in the millions, and the reaction mixtures become gels that cannot be stirred. Products isolated from the reactions cannot be processed by conventional thermoplastic means and are believed to be crosslinked. This behavior is attributed to the ability of unsaturated butadiene units in the polybutadiene segments of the polymer to alkylate polystyrene units in the polymers, and thereby cause branching or crosslinking reactions. This problem is particularly acute when nitration and arylsulfonylation reactions are attempted because branching occurs much faster than the desired nitration or arylsulfonylation reactions. The problem is not as severe when the substitution reaction occurs much faster than the branching reaction, as is the case when acylation reactions are used to modify hydrogenated thermoplastic elastomers.
Residual unsaturation which remains after hydrogenation of an unsaturated elastomers is undesirable and is a point of chemical instability. In the invention described herein, post-hydrogenation reactions are used to remove or significantly lower the amount of residual unsaturation in hydrogenated polymers and copolymers and thereby enhance the chemical resistance of the polymers. For example, as the following discussion will show, modification of Kraton.RTM. G-1652 by reaction with anisole, removes a large portion of the residual unsaturated butadiene units and thus enables the resulting polymer to be nitrated or arylsulfonylated without the difficulties that attend attempts to nitrate or arylsulfonate unmodified Kraton.RTM. G-1652. Since improved resistance to acid promoted branching and/or crosslinking has been demonstrated to result from post-hydrogenation treatment, it is inferred that improved resistance to chemical processes, including oxidation and ozonolysis also results from post-hydrogenation reaction.